Catalyst for and method of preparing cellulose esters

ABSTRACT

Disclosed is a catalyst composition and a method of preparing lower fatty acid esters of cellulose which comprises esterifying at a temperature of between about 75 DEG  C. and about 110 DEG  C. a cellulose compound having esterifiable hydroxyl groups with an esterifying bath comprising an organic acid anhydride, a diluent, and a catalyst comprising from about 25 to about 75% by weight sulfuric acid and about 75 to about 25% by weight phosphoric acid, the catalyst being used in amounts of from about 0.3 to about 0.8% by weight based on the weight of cellulose.

TECHNICAL FIELD

This invention relates to the preparation of lower fatty acid esters ofcellulose in which sulfuric acid and phosphoric acid are used asesterification catalysts.

BACKGROUND ART

In the usual preparation of lower fatty acid esters of cellulose,sulfuric acid is the catalyst employed. This acid is very effective inthat it promotes the desired esterification of the cellulose underrelatively mild conditions but it combines with cellulose and thecellulose ester obtained often contains sulfuric acid radicals.Ordinarily these sulfate groups can be removed from the final product bya prolonged hydrolysis under very carefully controlled conditions.However, even after this treatment, the product may contain sufficientcombined sulfuric acid to render it unstable to prolonged heating atelevated temperatures such as may be encountered in molding operationsor the like.

In order to eliminate this disadvantage which may be encountered whensulfuric acid catalyst is employed, various so-called "non-combining"acids have been suggested as catalysts in cellulose esterificationprocesses. Perchloric acid is an example of such an acid catalyst.However, perchloric acid has several disadvantages when used as acatalyst. For instance it is corrosive on stainless steel equipment. Itis explosive in contact with readily oxidizable materials.

According to U.S. Pat. No. 2,861,069, other strong acid catalysts suchas phosphoric acid, hydrochloric acid, methane sulfonic acid, toluenesulfonic acid and the like have also been disclosed as usefulnoncombining catalysts in the esterification of cellulose. In addition,weak salts of strong acids such as zinc chloride, ammonium sulfates andthe like have been suggested.

Commercial cellulose esterifications, for the most part, areconsequently conducted using relatively large quantities of sulfuricacid as the reaction catalyst. Sulfuric acid is a very strong acidcatalyst for cellulose esterification and requires considerable reactiontemperature manipulation (cooling) in order to avoid excessivedegradation (reduction) of the cellulose molecular weight. Sulfuric acidalso combines with the cellulose during esterification to form acellulose sulfate ester which decreases the thermal stability of thecellulose organic ester. Other cellulose esterification catalysts suchas perchloric acid, hydrochloric acid, and zinc chloride are well knownin the art as noncombining catalysts. However, these catalysts areeither too corrosive (perchloric and hydrochloric acid) or too much (50to 100% based on weight of celluose) is required for efficient reaction(zinc chloride).

Certain sulfonic acids, e.g., para-toluene sulfonic acid, also are knownin the art as cellulose esterification catalysts but again large amounts(up to 50 wt. % based on cellulose) of catalyst are required at thenormal reaction temperature (40°-50° C.) and high quality, highα-cellulose content pulps are required to obtain an acceptable qualityproduct.

The present invention is believed to be the first realization that verysmall amounts of a catalyst mixture comprised of sulfuric and phosphoricacids may be used at reaction temperatures of 75°-110° C. to obtainacceptable quality cellulose acetate from low α-cellulose (e.g., viscosegrade) wood pulps.

DISCLOSURE OF INVENTION

It has now been found that if the sulfuric acid catalyst is used onconjunction with a small amount of phosphoric acid, and the sulfuricacid is used in small quantities, the esterification reaction is morecontrollable, reproducible, and smoother. Also, the cellulose esterproduct has improved (less) color and retains its viscosity better.

The present invention provides a catalyst composition and method ofpreparing lower fatty acid esters of cellulose which comprisesesterifying at a temperature of between about 75° C. and about 110° C.(preferably about 85°-95° C.) a cellulose compound having esterifiablehydroxyl groups with an esterifying bath comprising an organic acidanhydride, a diluent, and a catalyst. The catalyst comprises from about0.3% to about 0.8% by weight (preferably about 0.5%), based on theweight of cellulose, of a catalyst comprising a mixture of sulfuric andphosphoric acids. The weight ratio of the bath to the cellulose isbetween about 5:1 to about 10:1. Use of small amounts to catalyst andrelatively high esterification temperature is found to be especiallyadvantageous.

BEST MODE FOR CARRYING OUT THE INVENTION

According to the present invention, small amounts of particular sulfonicacid catalysts are used in the esterification of cellulose allowing therection temperature to rise to between about 75° C. and about 110° C.Such reaction temperatures are advantageous, because as the exothermicreaction causes a rise in temperature, less cooling is required than inprocesses where the reaction temperature must be lowered a greateramount.

The catalysts are used in amounts of from about 0.3% to about 0.8%,based on the weight of cellulose. Preferably, about 0.5% based on theweight of cellulose, if used. The sulfuric and phosphoric acids aremixed in ratios of about 1:3 to about 3:1 for best results.

The addition of phosphoric acid to the sulfuric acid for hightemperature acetylation provides a more controllable, reproducible,smoother acetylation reaction with improved (less) product color andbetter retention of product viscosity during the high temperatureprocesses. The phosphoric acid may be added to the cellulose prior to orsimultaneously with the sulfuric acid. It is preferred to pretreat thecellulose with this catalyst mixture 20-30 minutes at 25°-30° C. priorto the high temperature acetylation reaction. Such a pretreatmentprocess ensures uniform distribution of the catalyst mixture throughoutthe cellulose polymer.

The high temperature acetylation process with sulfuric and phosphoricacid catalyst allows a lower liquid/cellulose ratio during reactioncompared to the conventional process thus enabling considerable savingsin liquid raw materials and recovery energy. Normally theliquid/cellulose ratio may be reduced from about 7.0-7.5:1 to about5.5-6:1.

Although the process according to this invention is especially useful inthe acetylation of low α-cellulose, such as viscose grade cellulose woodpulp, it may be used in the acetylation of any of the conventional typesof cellulose.

The catalysts in accordance with the invention are also useful inesterification processes which involve the esterification of cellulosecompounds containing free and esterifiable hydroxyl groups. Forinstance, partially substituted cellulose esters or cellulose ethers canbe esterified by lower fatty acid anhydrides and the reaction catalyzedas described herein. Sone cellulose esters to the preparation of whichthe use of catalysts described herein is particularly adapted arecellulose acetate, cellulose acetate propionate, cellulose acetatebutyrate, cellulose propionate and cellulose butyrate.

In the acetylation process as practiced in this invention, the cellulosewood pulp is esterified in a mixture of acetic and/or butyric acid(preferably about 2 to 3 parts per part of cellulose); and lower fattyacid anhydride (about 3 parts resulting in about 5-6 parts total liquidper part of cellulose, and 10 parts per part of cellulose, 2.5 to 3.0parts preferred) using the catalyst system described herein. Although itis preferred that the liquid bath/cellulose ratio be about 5 to 6 to 1,larger amounts of liquid bath may sometimes be used to increase thisratio up to about 10 to 1. The reaction is carried out at temperaturesfrom 75° to 110° C. (85°-95° C. preferred) until a grain-free, smoothreaction solution is obtained. Sufficient aqueous acetic acid may beadded to destroy the excess acetic anhydride and to provide water in thereaction solution for hydrolysis which is carried out at about 100° C.,until the desired acetyl content is obtained on the cellulose acetate.In the preferred process, one-half of the sulfonic acid catalyst isneutralized with sodium acetate prior to hydrolysis at about 100° C. Thecellulose acetate, after hydrolysis, is precipitated, washed,stabilized, and dried according to methods well known in the art.

In the high temperature acetylation process as described in thisinvention, the cellulose pulp does not require any special activationwith water, aqueous acetic acid, or acid catalyst in order to achieveadequate reactivity. It is to be understood that the process of thisinvention is applicable to high quality, high α-cellulose content woodpulps, but is especially useful in connection with the low α-cellulosepulps as described in the examples cited herein.

The following examples are submitted for a better understanding of theinvention. Conventional esterification techniques are used with thecatalysts according to this invention as described above.

    ______________________________________                                                   Exam- Exam-   Exam-   Exam- Exam-                                             ple 1 ple 2   ple 3   ple 4 ple 5                                  ______________________________________                                        Sulfuric/Phosphoric                                                           Acid Ratio   1/              0.75/       1.0/                                 (by wt.)     0.75    1/2     1.0   1/2   0.75                                 Catalyst Wt.                                                                  % Based on                                                                    Cellulose    0.5     0.5     0.5   0.5   0.5                                  Reaction Temp. °C.                                                                  97°-98°                                                                 97°-98°                                                                 95°-96°                                                               97°-98°                                                               95°-96°                Reaction Time, Hrs.                                                                        2.0     2       3.0   3     2.5                                  .sup.a Acetyl, %                                                                           39.8    39.3    39.8  39.9  38.7                                 Intrinsic Viscosity                                                                        1.61    1.56    1.76  1.69  1.89                                 Sulfur, %    0.0026  0.0016  0.0024                                                                              0.0015                                                                              0.0023                               ______________________________________                                         .sup.a After 8-10 Hrs. Hydrolysis at 103 ± 1° C. in 85% Aqueous     Acetic Acid                                                              

Inherent viscosity (I.V.) is measured by dissolving 0.25 g of celluloseester in 100 ml of acetone at 25°+0.1° C.

Unless otherwise specified, all parts, percentages, ratios, etc., are ona weight basis.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

We claim:
 1. A method of preparing lower fatty acid esters of cellulosewhich comprises esterifying at a temperature of between about 75° C. andabout 110° C. a cellulose compound having esterifiable hydroxyl groupswith an esterifying bath comprising an organic acid anhydride, adiluent, and from 0.3 to 0.8% by weight based on the weight of celluloseof a catalyst comprising (a) from about 25 to about 75% by weight ofsulfuric acid and (b) from about 75% to about 25% by weight ofphosphoric acid, the weight ratio of said bath to the cellulose beingbetween about 5:1 and about 10:1.
 2. A method according to claim 1wherein the esterification is carried out at a temperature of about85°-95° C.
 3. A method according to claim 2 wherein said sulfuric acidis present in an amount of about 0.20%, based on the weight ofcellulose, and said phosphoric acid is present in an amount of about0.07-0.6% based on the weight of cellulose.
 4. A method according toclaim 1 wherein the weight ratio of said bath to said cellulose isbetween about 5:1 and about 6:1.
 5. A method according to claim 1wherein the ester of cellulose is acetate.
 6. A method of preparinglower fatty acid ester of cellulose which comprises esterifying at atemperature of between about 85° C. and about 95° C. a cellulosecompound having esterifiable hydroxyl groups with an esterifying bathcomprising an organic acid anhydride, a diluent, and a catalystcomprising (a) from about 0.2% by weight, based on the weight ofcellulose, of sulfuric acid and (b) from about 0.07 to about 0.6% byweight, based on the weight of cellulose, of phosphoric acid, the weightratio of said bath to the cellulose being between about 5:1 and about10:1.